The use of titanium dioxide in so many application areas such as pigment in paint and plastic compositions is a direct result of the many differing and useful properties of the pigment. A common problem with the uses of titanium dioxide is that the pigments can either promote the reaction of polymer additives with each other, or react directly with one or more of the additives. These reactions can lead to discoloration problems. In addition, titanium dioxide is photoreactive. Exposure to UV radiation can result in the generation of free radicals on the surface of titanium dioxide particles. The free radicals can migrate to the film-forming component of the composition, resulting in degradation or failure of the film. The employment of titanium dioxide in pigmented plastics, coatings, papers, and fiber compositions, therefore, can induce a photoactivity that results in oxidative degradation, which is destructive to the materials of which they are a part.
Therefore, minimizing the migration of free radicals is desirable in order to provide lightfast and stable pigments for plastic and coating applications. As a consequence, most titanium dioxide pigments are provided with some form of surface coating during manufacture to promote dispersibility and to reduce photoactivity. Widely used approaches to overcome this problem involve coating titanium dioxide particles with hydrous oxide silica, alumina, zirconia, phosphate, or mixtures thereof. These materials can be coated onto the surfaces of pigment particles in the range of 3 to 10 percent, by weight of titanium dioxide. This results in lacing. Lacing occurs as a result of volatiles being released from the pigment during high temperature poly-olefin fabrication processes. Lacing may also be attributable to titanium dioxide concentrates picking up moisture. A further disadvantage is that higher loadings of titanium dioxide pigment in a polymer concentrate result in slower processing rates.
Since titanium dioxide is photoactive, and can promote degradation of resin systems, hydrous oxide coatings are applied to titanium dioxide particles to improve durability, as mentioned above. Typically, hydrous oxide coatings on titanium dioxide particles are prepared by wet chemical methods. These involve precipitation of the hydrous oxides, such as silica, alumina, and zirconia, from solution. While these processes do provide somewhat durable coatings on the titanium dioxide particles, they often result in uneven, non-uniform, and porous coatings.
The surface treatment and coating of titanium dioxide are done to achieve improved performance characteristics in plastic compositions. Dispersibility, processability, and good optical properties are typically achieved through surface treatment of the pigment, whereas durability, low chemical reactivity, and low photochemical reactivity are typically obtained by coating the pigment with silica, alumina, and other metal oxide coatings. When precipitated using prior art practices, these metal oxide coatings tend to make the pigments more difficult to disperse in plastics. Also, pigments coated with metal oxides tend to have more adsorbed water which may cause out-gassing and lacing problems in production of plastics with coated pigments. Typically, these problems have not been solved with a single pigment. Thus, plastic compounders have generally had to choose between durability, which can be achieved with coated pigments, and dispersibility, processibility, and good optical properties which can be achieved with uncoated but treated or untreated pigments.
Titanium dioxide pigments and other pigments normally produced and used in the paint, plastics, or paper industry, are generally in the form of a finely divided powder. The powders are usually jet-milled or micronized, as a final step in their production. Jet-milling contributes dispersibility and gloss, but is also an energy-intensive and expensive step.
Jet-milled powders are inherently dusty and exhibit poor flow characteristics. Although free-flowing powders with low dusting can be obtained by spray drying, these powders generally exhibit poor pigmentary properties. Thus, pigment end users have generally had to choose between free-flowing, low dusting, spray dried pigments having poor pigmentary properties, and dusty, jet-milled pigments having poor flow characteristics.
Inorganic pigments, such as titanium dioxide, for example, are used in a variety of thermoplastic resin-based consumer goods. It is known, however, that the quality of the properties that the inorganic pigment imparts to the resin, such as opacity, color, brightness, UV stabilization, thermal stabilization, and the like, depend significantly on how evenly and uniformly the pigment is dispersed in the resin, as well as on the amount of pigment that can be effectively incorporated into the resin. It is also known that the hydrophobic nature of such thermoplastic resins is incompatible with the hydrophilic nature of such inorganic pigments, thus making it difficult to obtain good dispersion of the pigments in the resins. This is especially true at high pigment concentrations greater than 50 percent by weight, as pointed out in U.S. Pat. No. 4,183,843.
In the manufacture of titanium dioxide, it is difficult to produce a product that is low dusting, free-flowing, dense, and easily dispersed in paints and plastic melts.
Various types of coatings with hydrous metal oxides have been applied to titanium dioxide pigments to enhance the weathering character of the pigments and various processes have been developed for applying such coatings. In general, many previous processes involve the addition of a water-soluble hydrolysable metal salt to an aqueous slurry of titanium dioxide, and the addition of a pH adjusting agent to neutralize the slurry, and to form insoluble hydrous metal oxides on the pigment. This need not ensure a homogenous and uniform coating on the surface due to non-uniform and improper dispersion of the additive in the body of the aqueous titanium dioxide slurry.
Various problems have been encountered in processing titanium dioxide pigments having a high titanium dioxide content, which are used in plastics and which have been coated via prior processes. More particularly, various problems have been encountered in the filtration of such coated, plastic grade pigments, and in micronizing or milling such coated, plastic grade pigments. For example, the pigment is sticky and tends to stick to the filtration and milling apparatus. The sticking of the pigment to the various apparatus makes it extremely difficult to load the pigment into the milling apparatus and to remove the pigment after the milling operation. The finished pigment also often contains unacceptable levels of grit.
Polymeric compounds that are used to make plastics articles are processed at high temperatures for a number of industrial processes. At elevated temperatures, inorganic compounds such as titania pigments that have been incorporated in the polymeric compounds, can release volatile species such as water molecules, which can cause defects in polymers such as bubbles or holes. Holes formed in thin films may be referred to as “lacing,” which is a phenomenon that limits the operating conditions of the film manufacturer. Consequently, film manufacturers often desire titanium dioxide pigment that have or reduced tendency to cause lacing.
In response to the demands of customers for grades of titania that can be processed in polymers at increasing temperatures and pigment loadings, manufacturers have been reducing the amount of inorganic surface treatment. The reason is that the inorganic species are usually hydrated, either as part of the chemical structure or water adsorbed from the atmosphere onto the surface of the pigment. Upon heating, this water is driven off and is one of the key contributors to volatiles and subsequent defects such as lacing. Reducing the inorganic surface treatment reduces the total moisture content and reduces the volume of volatile species driven off of the pigment at elevated temperature. However, providing the titanium dioxide particles with a homogeneous, uniform, thin layer surface coating is difficult and results in uncoated patches on the surface of titanium dioxide particles.
Titanium dioxide is the premier white pigment used for whitening, brightening, and opacifying plastics, paper, and paints. As normally produced, titanium dioxide is a hydrophilic pigment, meaning that it is readily wet by water and not wet by hydrophobic materials like organic polymers. In order to permit titanium dioxide pigment to be wet-out by and dispersed in organic polymers, the surface of the pigment must be modified or made hydrophobic, so that the polymer will spread over the pigment's surface and good adhesion between the pigment and polymer will occur. Hydrophobic titanium dioxide pigments are prepared by treatment with “non-reactive” organic substances or “reactive” organic compounds.
It can be seen from the above discussion that the customers who process titanium dioxide pigments in polymers demand titanium dioxide pigments meeting the following characteristics:
1. Good optical properties;
2. Good durability;
3. Resistant to chemical and photochemical reactions;
4. Anti-yellowing;
5. Low-volatility;
6. Low dusting; and
7. Good Processibility
There is a continuing need for a process for preparing improved pigments which meet all of the above requirements.